As-worked, heat treated cold-workable hypoeutectoid steel

ABSTRACT

Hypoeutectoid steels are worked within a temperature range of between the A1 temperature to 150* F. below the A1 temperature. The cross-sectional area of the steels is reduced by not less than 60% during working. After working, the steels can be heated to about the A1 temperature to obtain the optimum hardness and ductility for cold-working. The as-worked structure and the heat treated structure are also described.

United States Patent Kranenberg [4 July 1, 1975 [54] AS-WORKED, HEATTREATED 2,025,016 l2/l935 Chancellor l48/l2 2,814,578 ll/l957 White SF THYPOEUTECTOID 3,076,361 2/l963 Epstein et a1. 3,285,789 11/1966 Grangeet al. [75] Inventor; Helmut Krangnberg, Bethlghem, Pa 3,459,599 8/1969Grange l48/l2 73 A :Bthlh s: m ration, 1 sslgnee z z x orpo PrimaryExaminerC. Lovell Attorney, Agent, or FirmJ0seph J. OKeefe; CharlesFlledi y 1973 A. Wilkinson; John S. Simitz [2]] Appl. No.: 358,751

Related US. Application Data [57] ABSTRACT [62] Division of Ser. No.242,473, April 10, 1972, Pat. Hypoeutectoid steels are worked within atemperature 3,762,964- range of between the A, temperature to 150 F.below the A, temperature. The cross-sectional area of the [52] US. Cl.[48/36 steels is reduced by not less than 60% during working, [51] Int.Cl C22C 39/00; C2ld 7/14 After working, the steels can be heated toabout the Field of Search u A temperature to obtain the optimum hardnessand I48/ 14 ductility for cold-working. The as-worked structure and theheat treated [56] Reterences Cited structure are also described.

UNITED STATES PATENTS 2/l9l9 Murray l48/12 2 Claims, 6 Drawing FiguresFN EU JUL PAT SHEEI PATENTEDJIII I TEMPERATURE "F VICKERS HARDNESS N0.

OF THE INVENTION UNG CYCLE I 300 ENDOF PRIOR ART SPHEROIDIZED I200 ENDOF ANNEALING CYCLE HEAT TREATING CYCLE I 00 OF THE INVENTION l l l I l 02 4 s 8 I0 I2 I4 l6 l8 ANNEALING TIME hrs.

HARDNESS OF WORKED sTEEI 2 I 0 0 WHEN HEAT TREATED BY CYCLE OF THEINvENTIoN HARDNESS OF CONVENTIONALLY o WORKED STEEL WHEN SPHEROIDIZEANNEALED BY CONVENTIONAL CYCLE I80 I I70 i 60 I50 0 2 4 6 0 I0 I2 I4 l6l8 ANNEALING TIME hrs.

AS-WORKED, HEAT TREATED COLD-WORKABLE HYPOEUTECTOID STEEL This is adivision of application Ser. No. 242,473, filed Apr. l0, 1972, now US.Pat. No. 3,762,964.

BACKGROUND OF THE INVENTION This invention is in general directed to amethod for working hypoeutectoid steels within a temperature range toeffect a desired reduction in cross-sectional area. The hypoeutectoidsteels can be heated to a temperature for a time to obtain optimumhardness and ductility suitable for cold-working. More specifically, theinvention is directed to a method for working and heat treatinghypoeutectoid steels, for example, steels containing about 0.30 to about0.80% carbon, wherein said steels are worked to reduce thecross-sectional area by not less than 60% within a temperature range ofabout the A, temperature to about 150 F. below the A, temperature. Thesteels are subsequently heated to about the A, temperature to obtain ahardness and ductility suitable for cold-working, such as cold-headingand the like.

Steels used for cold working into various products, such as bolts,screws and the like, generally are of the hypoeutectoid type containingup to about 0.80% carbon.

Prior art practice in manufacturing cold-workable hypoeutectoid steelsis to refine the steel in a metallurgical furnace, such as an electricfurnace and the like, tap and teem the steels into ingot molds to formingots. The ingots are hot-worked at an austenitizing temperature intothe end product, such as bars, billets, rods, wire and the like. The endproduct is slow cooled to ambient temperature. The steels arespheroidize annealed to obtain a uniform structure which issubstantially completely spheroidized; is substantially completely freeof a carbide network and pearlite; and is relatively soft and ductile.Because of the duplex structure and high hardness of the steels afterslow-cooling from the hot-workin g temperatures, the spheroidize annealcycles which are used are lengthy. The steels must be soaked at theproper temperature for from about hours to days to completely andeffectively produce the desired microstructure and hardness andductility required for good cold formability. In order to decrease thetime of annealing, the steels are alternately heated and cooled to a fewdegrees of temperature above and below the A, temperature several times.Although the microstructure and hardness produced by the practices areacceptable by present day standards, the cyclic heating and coolingpractice does not effectively reduce the length of the annealing cycle.Then, too, the microstructure of hypoeutectoid steels treated by thecyclic heating and cooling method can contain evi dence of lamellarcarbides. The hardness of the steels is reduced to a hardness suitablefor cold-working after lengthy time at the spheroidize annealingtemperature but maximum reduction in hardness may not be achieved.

Recently several improvements in the manufacture of cold-workable steelshave been suggested. One such improvement is US. Pat. No. 3,285,789issued Nov. 15, i966 to Raymond A. Grange et al. titled Method ofSoftening Steel. The improvement is directed to heating hypoeutectoidsteels to temperatures wherein the steels are completely austenitic,working the steels while at these temperatures, cooling the steels toambient temperature and spheroidize annealing the worked steels within atemperature range between the A, temperature to 50 F. below the A,temperature. The structure obtained after spheroidize annealing issatisfactory by present standards. Complete spheroidization and theelimination of lamellar carbides is not achieved in the asworkedcondition.

The as-worked steels contain ferrite and pearlite, therefore the steelsmust be spheroidize annealed to produce a spheroidized structure.

Another improved method is directed to hypereutectoid steels and highlyalloyed steels as described in U.S. Pat. No. 3,459,599 issued Aug. 5,1969 to Raymond E. Grange titled Method of Thermomechanically AnnealingSteel." Hypereutectoid steels are heated to and drastically worked at atemperature not more than l50 F. above the A temperature and orefinished below the A, temperature but not more than 50 F. below the A,temperature. The method, while applicable to hypereutectoid steels, isnot applicable to hypoeutectoid steels. The problems of completespheroidization with elimination of lamellar carbides connected withhypoeutectoid steels is not solved.

Although prior art practices have been developed to roll steelsferritically as disclosed in US. Pat. No. 3,076,361 issued Feb. 5, l963to S. Epstein et al. titled Rolling Steel in Ferritic State, the highalloy and tool steels to which the process is directed, must be heattreated, for example, spheroidize annealed, prior to heating andworking. The special heat treatment prior to heating for working andcontrolled heating for work ing increase the cost of the production ofthe steel.

SUMMARY OF THE INVENTION It is an object of this invention to provide amethod for producing hypoeutectoid steels having good formabilitywherein said steels are worked within a temperature range below the A,temperature and the worked steels are heat treated at about the A,temperature for a time to decrease the hardness and increase theductility of the steels.

It is an object of this invention to provide a method for producinghypoeutectoid steels suitable for coldworking wherein said steels areworked within a temperature range between about the A, temperature toabout F. below the A, temperature and the worked steels are heat treatedat about the A, temperature for a time to obtain a desired hardness andductility.

It is an object of this invention to provide a method for producingas-worked hypoeutectoid steels wherein said steels are worked within atemperature range between about the A, temperature to about 150 F. belowthe A, temperature to obtain a microstructure of fine well-dispersedspheroidal carbides in a fine ferritic matrix substantially devoid oflamellar carbides.

It is an object of this invention to provide a method for producinghypoeutectoid steels suitable for coldworking, said steels containingabout 0.30 to about 0.80% carbon, wherein said steels are worked withina temperature range between about the A, temperature to l50 F. below theA, temperature for a time to reduce the cross-sectional area thereof bynot less than 60% and the worked steels are heat treated at about the A,temperature for up to about six hours to produce a microstructureconsisting of well-dispersed spheroidal carbides in a ferritic matrix,said steels being characterized by low hardness and good ductility.

Broadly, the invention includes working hypoeutectoid steels within atemperature range of about the A, temperature to about 150 F. below theA, temperature to reduce the cross-sectional area by not less than 60%and to heat treat the steels at about the A, temperature for a time toobtain a low hardness and ductility.

DESCRIPTION OF THE DRAWINGS FIG. I is a reproduction of aphotomicrograph taken at 2200 diameters of a hypoeutectoid steelas-worked by the method of the invention.

FIG. 2 is a reproduction of a photomicrograph taken at 2200 diameters ofa hypoeutectoid steel as-worked and heat treated by the method of theinvention.

FIG. 3 is a reproduction of a photomicrograph taken at 500 diameters ofa hypoeutectoid steel as-worked and heat treated by the method of theinvention.

FIG. 4 is a reproduction of a photomicrograph taken at 500 diameters ofa hypoeutectoid steel as-worked and spheroidize annealed by aconventional method.

FIG. 5 is a graph comparing a conventional method of spheroidizeannealing and the method of heat treating of the invention.

FIG. 6 is a graph showing a comparison of the decrease in hardness ofsteels worked and spheroidize annealed by a conventional method and bythe method of the invention.

PREFERRED EMBODIMENT OF THE INVENTION l-Iypoeutectoid steels suitablefor cold-working, for example. cold-forming, cold-threading and thelike, can be made by working the steels within a temperature range belowabout the A, temperature and heat treating the worked steels at aboutthe A, temperature for a time to obtain optimum hardness and ductility.The as-worked hypoeutectoid steels have a microstructure of finespheroidal carbides well-dispersed in a fine ferritic matrixsubstantially devoid of lamellar carbides. The heat treated steels havea microstructure of somewhat larger spheroidal carbides well dispersedin a ferritic matrix substantially devoid of lamellar carbides.

In the practice of the invention, hypoeutectoid steels are melted andrefined in any type of metallurgical furnace, such as basic oxygenfurnace, electric furnace, open-hearth and the like. The refined steelsare tapped into a ladle and teemed into ingot molds in the conventionalmanner. The ingots thus formed are heated to an austenitizingtemperature and are rolled into billets and cooled to black. At thisstage of processing the steels, it is possible to pursue either one oftwo steps: (1) the billets can be heated to an austenitizing temperatureand worked at the austenitizing temperature to effect a reduction in thecross-sectional area, said reduction is of such a nature that the steelswill require additional working at a temperature to elfect at leastanother 60% reduction in cross-sectional area to obtain the final sizedesired, after which the billets are cooled rapidly, for example, in airthrough the A ,A, temperature range to a temperature range between theA, temperature and 150 F. below the A, temperature, in which temperaturerange the additional reduction in crosssectional area is achieved, or(2) the billets can be reheated to a tempeature range between about theA, temperature and 150 F. below the A, temperature and the steels workedwithin this temperature range to the desired final size. Whichever ofthe two above steps is taken, the hypoeutectoid steels are worked withina temperature range of between about the A, temperature to 150 F. belowthe A, temperature to obtain not less than 60% reduction incross-sectional area to achieve the results of the invention. Afterworking, the hypoeutectoid steels can be heat treated for a time atabout the A, temperature. It will be understood that wherever heattreatment is used in the specifications and claims in regards to thesteels processed by the method of the invention such heat treatmentincludes heating the steels to about the A, temperature for a time toreduce the hardness of the steels and to in crease the ductility of thesteels with little or no effect on the spheroidization microstructure ofthe steels other than a slight increase in the size of the carbides andferritic matrix. It will also be understood that to raise thetemperature of the steels to the working temperature range between aboutthe A, temperature and 150 F. below the A, temperature it is possible toheat the steels above the A, temperature or even above the A,temperature so long as the steels are cooled to within the workingtemperature range described above before any reduction incross-sectional area is started. Although working the steels within thetemperature range of about A, temperature to about 150 F. below the A,temperature will achieve the results of the invention, good results canbe achieved by working the steels within a temperature range of about 5F. to about F. below the A, temperature and better results can beachieved by working the steels within a temperature range of about 75 F.to about I50 F. below the A, temperature. It is, therefore, preferred towork the steels within a temperature range of about 75 F. to about F.below the A, temperature.

The hypoeutectoid steels are worked within the temperature rangesmentioned above to obtain a reduction in the cross-sectional area of notless than 60%. Samples of the steels asworked by the above describedmethod were examined by electron microscopy at a magnification of 2200diameters. The microstructure was found to consist of fine spheroidalcarbides welldispersed in a fine-grain ferritic matrix. A reproductionof an electron photomicrograph of the structure at the lattermagnification is shown in FIG. I. It can be seen that the carbides arewell spheroidized and are less than 1 micron in size when compared witha line 5 microns long drawn on the lower right-hand corner of theelectron photomicrograph for comparison purposes. The ferritic grainsare also small, not more than 1.5 microns in size, although they appearto be very large when compared to the carbides. The microstructure issubstantially free of lamellar carbides.

The hypoeutectoid steels were heat treated at about the A, temperaturefor from about three hours to about six hours. A reproduction of anelectron photomicrograph of a sample of the steels after heat treatmenttaken as a magnification of 2200 diameters is shown in FIG. 2. Thespheroidal carbides and ferrite grains have been coarsened by the heattreatment. The carbides are less than 5 microns in size when compared toa line 5 microns in length drawn in the lower right-hand corner of thephotomicrograph for comparison purposes.

A microscopic examination at a magnification of 500 diameters of thesteels after heat treatment is shown in FIG. 3. The microstructure canbe seen to consist of finely divided spheroidal carbides well dispersedin a ferritic matrix. The microstructure is substantially free ofcarbide network, and lamellar carbides.

A microstructure at a magnification of 500 diameters typical ofhypoeutectoid steels processed by a conventional method of producinghypoeutectoid steels, that is, hot rolling at austenitizingtemperatures, for example, 1550 F., and spheroidize annealing by aconventional annealing cycle wherein the steels are cyclically heatedand cooled a few degrees in temperature above and below the A,temperature for about 17 hours, is shown for comparison purposes in FIG.4. The carbides are tending to spheroidize but a large portion thereofretain lamellar-like formations and are not weleldispersed. Ferritegrains are outlined by the carbides. The ferrite grains appear to belarger in the conventionally worked and spheroidize annealed steels thanthe ferrite grains of the steels worked and heat treated by the methodof the invention shown in FIG. 3.

The steels of the invention were tested for hardness both in theas-rolled and heat treated condition. The as-rolled steels with amicrostructure shown previously in FIG. 1 were found to have a hardnessof 200 DPH (Vickers) to about 230 DPH (Vickers) which is equivalent to ahardness within a range of about 190 BHN to about 220 BHN. The hardnessrange is above the hardness desired in steels which are to becold-worked. After heat treating for a time, about three hours to aboutsix hours, by the method of the invention, the steels had been loweredin hardness by about 80 points in both DPH (Vickers) and BI-IN, which iswell within the hardness range for cold-working the steels, for example,cold-heading.

A comparison of the short heat treating cycle of the invention and atypical conventional spheroidize annealing cycle is shown in FIG. 5.Note that the heat treating cycle of the invention is considerablyshorter than the typical conventional spheroidize annealing cycle.

FIG. 6 is a comparison of the effect of the heat treating cycle of theinvention on the as-worked hardness of hypoeutectoid steels and atypical conventional spheroidize annealing cycle in the as-workedhardness of the hypereutectoid steels. It will be noted that theasworked hardness of the hypoeutectoid steels of the invention washigher than the as-worked hardness of hypoeutectoid steels prepared by aconventional hot working process. However, the hardness of thehypoeutectoid steels worked by the method of the invention decreasedmuch more rapidly when heat treated than the hardness of thehypoeutectoid steels worked by conventional hot working process. Infact, the hardness of the hypoeutectoid steels of the invention afterheat treating at about the A, temperature for about six hours iscomparable to the hardness of hypoeutectoid steels hot rolled byconventional hot rolling and spheroidize annealed by conventionalannealing cycle for about 17 hours. As seen in the dotted line, thehardness of the hypoeutectoid steels of the invention is loweredslightly when heat treated for longer periods of time. At each intervalof time the hardness of the hypoeutectoid steels of the invention islower than the hypoeutectoid steels prepared by a conventional hotrolling and spheroidize annealing cycle.

The as-worked hardness of the hypoeutectoid steels of the invention maybe sufficiently high to preclude cold forming, however the tensilestrength and reduction in area of these steels are better thanconventionally processed steels of the same grade. Therefore, in someapplication, the hypoeutectoid steels of the invention can be used inthe as-worked condition.

It will be understood that wherever percentages are mentioned in thesespecifications and claims, such percentages are on a weight basis unlessotherwise noted.

In a specific example of the invention, a hypoeutectoid steel having achemical analysis of: carbon 0.39%, manganese 0.75%, phosphorus 0.0l7%,sulfur 0.022%, silicon 0.18% was prepared in a basic oxygen furnace. Thesteel was melted, poured and teemed into 34 inches (1) ingot molds. Theingots were bloomed to 4 inches by 4 inches square billets and cooled toambient temperature. The billets were reheated to austenitizingtemperature and reduced in size to Z-Vs inches X l- /z inches billets,finished at 1900 F. and allowed to drop in temperature to about 11 200F. in air. The billets were reduced in cross-sectional area by 60.4% tol-% inches in diameter. The rounds were air cooled to ambienttemperature. Microscopic examination at a magnification of 22,000diameters of samples cut from the bars showed a microstructure of fine,well-dispersed spheroidal carbides of about 0.1 to 0.3 microns in sizein a fine-grained ferritic matrix of about 0.5 to L5 microns in size,devoid of lamellar carbides. The bars were heated in a furnace at atemperature of about 1 300 F. for five hours. The bars were slow cooledto room temperature.

Microscopic examination at a magnification of 8200 diameters of thesteel after heat treatment disclosed a microstructure of well-dispersedspheroids of carbides of about .5 to 2.5 microns in size in a ferriticmatrix of about 3 to l0 microns in size. The hardness of the asrolledbars was 229 DPH (Vickers). After heat treating, the hardness was l44DPl-I (Vickers). Tensile tests of as-rolled bars showed the steels tohave a tensile strength of l0l,000 pounds per square inch and areduction-in-area of 68%. The reduction-in-area compares favorably tosteels processed by prior art methods which had a tensile strength of89,000 pounds per square inch and a reduction-in-area of 63%.

The bars were heat treated at l300 F. for five hours. Tensile testsshowed the steels to have a tensile strength of 73,000 pounds per squareinch and a reduction-inarea of 79%. The ductility of the bars preparedby the method of the invention had improved ductility as compared to theconventionally treated bars.

I claim:

1. A heat treated hypoeutectoid steel consisting essentially of about0.30 to about 0.80% carbon and the remainder iron and incidentalimpurities characterized by having good forrnability and amicrostructure comprising fine well-dispersed spheroidal carbides havinga size of not more than 0.3 microns in a ferritic matrix, said ferriticmatrix being free of lamellar carbides and having a grain size of aboutthree microns to about ten microns.

2. An as-worked hypoeutectoid steel characterized by having highstrength and good ductility, said steel consisting essentially of about0.30 to about 0.80% carbon and the remainder iron and incidentalimpurities having a microstructure comprising finely-dividedwelldispersed spheroidal carbides in a fine-grain ferritic matrix, saidcarbides being not more than 1 micron in size and said ferritic matrixhaving a grain size of not more than 1.5 microns.

1. A HEAT TREATED HYPOEUTECTOID STEEL CONSISTING ESSENTIALLY OF ABOUT0.30 TO ABOUT 0.80% CARBON AND THE REMAINDER IRON AND INCIDENTALIMPURITIES CHARACTERIZED BY HAVING GOOD FORMABILITY AND A MICROSTRUCTURECOMPRISING FINE WELL-DISPERSED SPHEROIDAL CARBIDES HAVING A SIZE OF NOTMORE THAN 0.3 MICRONS IN A FERRITIC MATRIX, SAID FERRITIC MATRIX BEINGFREE OF LAMELLAR CARBIDES AND HAVING A GRAIN SIZE OF ABOUT THREE MICRONSTO ABOUT TEN MICRONS.
 2. An as-worked hypoeutectoid steel characterizedby having high strength and good ductility, said steel consistingessentially of about 0.30 to about 0.80% carbon and the remainder ironand incidental impurities having a microstructure comprisingfinely-divided well-dispersed spheroidal carbides in a fine-grainferritic matrix, said carbides being not more than 1 micron in size andsaid ferritic matrix having a grain size of not more than 1.5 microns.